Field of the Invention
The present invention generally relates to a method for manufacturing a double pipe, such as a method for manufacturing a curved double pipe.
Description of the Related Art
Catalytic converters are utilized, e.g., in vehicles, to convert toxic pollutants, such as carbon monoxide, unburned hydrocarbons and nitrogen oxides, into less toxic or non-toxic substances, such as carbon dioxide, water and nitrogen gas. To perform this purification, catalytic converters utilize a “redox” (i.e. reduction and oxidation) catalyst (purification catalyst) that typically functions most optimally or efficiently at a predetermined temperature or higher. Therefore, to avoid a decrease in the temperature of an exhaust gas flowing from the internal combustion engine (or a similar fuel burning device) into the catalytic converter, an exhaust pipe between the internal combustion engine, etc. and the catalytic converter may include an inner-outer double pipe, in which an air space or clearance is provided between the inner and outer pipes. This air space or clearance serves as an insulator to minimize cooling of the exhaust gas as it flows through the inner pipe to the catalytic converter.
Exhaust pipes typically have a curved shape, because it is necessary to pass the exhaust pipe through narrow spaces underneath the vehicle so as to avoid interference with other parts of the engine, transmission, vehicle chassis, etc. However, if the curved exhaust pipe is formed simply by bending a straight double pipe, the curved portion becomes flattened, which means that a prescribed air space (clearance) can not be reliably and readily formed between the inner pipe and the outer pipe.
As one example for overcoming this flattening problem, Japanese Patent Laid-Open No. 2000-79417 discloses a method for forming a curved double pipe that includes filling a straight double pipe with water and then immersing it in pressurized liquid nitrogen to form ice inside the double pipe. In this frozen state, the entire double pipe is bent by performing a draw-bending process or a similar metal working technique. After the double pipe has been bent, the ice is melted and the water is drained.
Because it is necessary to quickly freeze the water to form the ice, the above-described known manufacturing method incurs additional equipment and material costs due to the use of pressurized liquid nitrogen. Moreover, it involves additional labor costs for the managing (procuring) and handling the liquid nitrogen.